1. Field of the Invention
This invention relates to a method and device of determining the concentration of analytes in sample using an extraction device whereby the concentration of analytes of interest can be determined from the diffusion coefficient for said analytes. More particularly, this invention relates to a method and device for determining the concentration of organic and inorganic compounds in liquid and gaseous samples.
2. Description of the Prior Art
It is known to use solid phase microextraction (SPME) and polydimethylsiloxane (PDMS) coated fibers to extract volatile organic compounds (VOC's) in environmental samples. PDMS is the most widely used coating for extracting nonpolar volatile analytes as well as many polar analytes. However, the sensitivity of mixed phase SPME coatings, such as PDMS/DVB and Carboxen/PDMS was reported to be much higher compared to PDMS coating for extracting VOC's (see Mani et al, Applications of Solid Phase Microextraction, RSC, Cornwall, U.K., 1999, Chapter 5). Mixed phase coatings have some complementary properties compared to PDMS and are more suitable for sampling highly volatile species (see Pawliszyn, Solid-Phase Microextraction: Theory and Practice, Wiley-VCH, Inc., New York, 1997, Chapter 4). Mixed phase SPME fibers have been used for sampling and quantifying target VOC's present in indoor air at the part per billion level and even at the part per trillium level.
Indoor air quality and its potential impact on human health is an increased concern to the public and government environmental agencies. Many VOC's, such as formaldehyde, aromatic compounds, and halogenated hydrocarbons have been found to be highly toxic to humans. Large-scale air quality testing by conventional air sampling methods can often be time-consuming and expensive. Solid phase microextraction coupled with gas chromatography has been previously successfully applied to analyze various air samples. Chai et al., Analyst, 1993, 118,1501 reported the determination of the presence of volatile chlorinated hydrocarbons in air by SPME in 1993. Martos et al. developed a new method using a linear temperature-programmed retention index method to calibrate SPME devices for fluctuations in sampling temperature, and for air analysis of total VOC's with (PDMS) fibers (see Martos et al., Analytical Chemistry, 1997, 69, 206 and 402). Grote et al used SPME for fast quantitative analysis of acetone, isoprene and ethanol in human breath with SPME fibers in Analytical Chemistry, 1997, 69, 587. It is known that the syringe-like SPME device is portable and can be easily used for field analysis. When coupled with a field-portable gas chromatograph, both SPME sampling and instrumental analysis can be conducted at the test site without the need for sample preservation (see Koziel et al, Analytical Chemistry, Acta, 1999, 400(1-3), 153.
In mixed phase coatings, the majority of interaction on porous polymer particles is determined by the adsorption process. With mixed phase coatings, the molecules can be attracted to a solid surface via van der Waals, dipole-dipole, and other weak intermolecular forces (see Górecki et al, Applications of Solid Phase Microextraction, RSC, Cornwall, UK, 1999, Chapter 7). Hydrophobic interaction and electrostatic interaction also occur when extracting analytes from water and ionizable analytes from aqueous phase, respectively. Compared to the diffusion coefficient in liquid coatings of PDMS or PA, the diffusion coefficients of VOC's in divinylbenzene and Carboxen are so small that, within the frame of SPME analysis, essentially all the molecules remain on the surface of the coating. Therefore, the fundamental difference between adsorption and absorption is that in adsorption molecules bind directly to the surface of a solid phase while, in absorption, they dissolve into the bulk of the liquid phase.
The Langmuir adsorption isotherm is one of most important adsorption theories. The Langmuir model assumes there is only a limited number of surface sites that can be occupied by analyte molecules, all sites are equivalent, and there is no interaction between absorbate molecules on adjacent sites. The Langmuir adsorption isotherm was used to describe the adsorption equilibrium on PDMS/DVB and Carbowax/DVB coatings. A linear function is found to exist only if the affinity of an analyte toward the coating is low or its concentration in the sample is very low. In a real sample matrix, for example, air, there are usually more than two components. Since different components have different affinities towards the active sites, the presence of multi-components must affect the adsorption of one other. Unlike the non-competitive absorption process in liquid coatings, adsorption process onto porous polymer coatings in a multi-component system is a competitive process and therefore displacement effect is expected. Sampling conditions, the sample matrix composition and concentration can largely affect the amount of analytes extracted by mixed phase fibers. From a practical point of view, this makes quantitative analysis using porous polymer SPME coatings more difficult.
The majority of adsorption models are based on the equilibrium theory. In SPME, however, the equilibrium time ranges from a few minutes to a couple of hours depending on the nature of the analytes and the sampling conditions (see Ai et al., Applications of Solid Phase Microextraction, RSC, Cornwall, UK, 1999, Chapter 2). For porous solid coatings, the equilibrium time for the same analyte is usually much longer than that in liquid coatings. It may be impractical to wait for partition equilibrium of all of analytes in the matrix if the equilibrium times for some analytes are too long.
In the direct SPME system, such as sampling in air or in water, the analyte movement proceeds in two steps. The first step consists of the mass transfer of analytes from the bulk sample matrix to the surface to the SPME polymer coating followed by diffusion of the analytes within the coating. Fick's first law of diffusion (equation 1) can describe the rate of mass diffusion in the sample matrix in the coating as follows:                     F        =                  -                                    D              s                        ⁡                          (                                                ∂                                      C                    s                                                                    ∂                  x                                            )                                                          (eq.  1)            
Where F is the flux of analyte in the direction x from the sample matrix bulk to the SPME fiber surface.                (i) Ds is the diffusion coefficient of the analyte in the sample matrix,        (ii) Cs is the analyte concentration in the sample bulk.        
In a static gas system, mass movement results only from molecular diffusion due to intermolecular collisions. In practice, both molecular diffusion and bulk fluid movement must be considered. The extent of fluid movement (agitation), reflects the access of analytes to the surface and is frequently described as a theoretical parameter called the boundary layer (Cooper et al, Air Pollution Control: A Design Approach, Waveland Press Inc., Prospect Heights, 1994, Chapter 13).
According to the boundary layer theory, a laminar sublayer or the sample matrix film is formed when a fluid passes a fixed object. The only way that the analyte can pass from the air bulk phase to the surface of the coating is via molecular diffusion across the boundary layer. In the liquid/solid interface, the thickness of the boundary layer is determined by the agitation conditions and the viscosity of the fluid (see Pawliszyn, Solid Phase Microextraction: Theory and Practice, Wiley/VCH, Inc., New York, 1997).
In a gas system, air wind velocity is a very important factor in mass transfer process. Because the value of wind velocity represents the degree of bulk air movement, wind velocity will influence the overall mass transfer rate in the bulk of fluid. Based on mass transfer theories, the mass transfer rate of an analyte is proportional to the mass diffusivity, and inversely proportional to the thickness of gas film at the interface.
Many factors such as temperature, pressure, molecular structure and molecular weight can directly affect the molecular diffusion coefficients of VOC's (see Lugg, G. A., Analytical Chemistry, 1968, 40 (7), 1072). Since accurate experimental measurement of the diffusion coefficient is difficult, relatively few values for organic compounds in gas systems are available from the literature. A number of methods have been proposed for estimation of diffusion coefficients of VOC's in air systems. The method by Fuller, Schettler and Giddings (FSG method) was reported to be most accurate for non-polar organic gases at low to moderate temperature (see Lyman et al, Handbook of Chemical Property Estimation Method, ACS, McGraw-Hill, Inc., New York, 1982, Chapter 17). Minimal error is associated with the aliphatics and aromatics. FSG model describes that the molecular diffusion coefficient of an analyte is directly proportional to temperature, and inversely proportioned to air pressure. The relative humidity of air is another factor that can affect VOC extraction on SPME fibers because water molecules participate in the adsorption process.